1. Field of the Invention
The present invention relates to a printer capable of printing on both sides of a piece of sheet (hereinafter, “a piece of sheer” may simply referred to as a sheet). The printer of the present invention is generic name of a device that transport cut sheets one after the other from a sheet case to a printing unit where characters, graphics, or photographs, etc., are sequentially printed on each sheet. It is not limited to a single-function printer and instead, a copier, a facsimile machine, a complex device (or a multifunction device) or the like that comprises a device for printing on a sheet corresponds to what we call a “printer” herein.
2. Description of the Related Art
Recently, in order to save resources, printers capable of printing on both sides of each sheet have been actively introduced into offices or homeplaces.
In general, a printer that prints on one side of a sheet has a cassette (or a sheet case), a printing unit, and a supply unit. Sheets not yet printed are accommodated in the cassette. The printing unit comprising a printing head which discharges ink droplets or toner on the sheet. The supply unit feeds a piece of sheet one by one from the cassette to the printing unit. Printing on both sides of sheets further requires a sheet returning mechanism for transporting sheet one side of which has been printed at the printing unit, to the printing unit once again. Japanese Patent Application Laid-Open No. 2004-102165 (see particularly FIG. 2) discloses one example of a printer having a sheet returning mechanism that enables both side printing (double face printing). This printer is configured as described below. On the lower side of a case of a printer, the cassette in which stacked sheets are contained is located. On the tipper side of the case, the printing unit comprising a toner type printing head that discharges ink droplets by means of an electro photograph process is located. Between the printing unit located above and the cassette located below, the supply unit is located. The supply unit feeds a sheet from the cassette to the printing unit. The supply unit feeds sheets one by one from the cassette toward the printing unit. A sheet fed from the cassette runs through a U-shaped feed-in pathway. The U-shaped feed-in pathway guides the sheet to the printing unit. The sheet guided to the printing unit is printed on one side by the printing unit. The one side printed sheet is discharged (or pulled out) from the printing unit through a feed-out pathway. The one side printed sheet once stops on the feed-out pathway. Then, the sheet is back-fed through the feed-out pathway and transported to return pathway that diverges from the feed-out pathway. The feed-out pathway diverged from the feed-out pathway extends downward along with the lateral side of the printing unit. The return pathway bends from the side on the printing unit toward above the cassette. The return pathway passes between the cassette and the printing unit, and joins into feed-in pathway. Along the return pathway, a number of rollers are arranged for transporting sheets. Passing through the return pathway and feed-in pathway, the one side printed sheet is fed again to the printing unit. The return pathway and the number of rollers arranged along with the return pathway forms sheet returning mechanisms that enable both side printing. Passing through the U-shaped feed-in pathway, sheets are reversed upside down and transported to the printing unit. In other words, the surface that is the top face of the sheet when the sheet is in the cassette becomes the underside when the sheet is in the printing unit. Passing through the U-shaped feed-in pathway once again, the one side printed sheet will have other side printed.